As a consequence of the growing integration density in the semiconductor industry, photolithography masks have to image increasingly smaller structures on wafers. In order to take account of this trend, the exposure wavelength of lithography apparatuses is being shifted to ever shorter wavelengths. Future lithography systems will probably operate with wavelengths in the extreme ultraviolet (EUV) range (preferably but not necessarily in the range of 6 nm to 15 nm). The EUV wavelength range places huge demands on the precision of optical elements in the beam path of future lithography systems. In all probability, the optical elements, and hence also the photolithographic masks, will be reflective optical elements.
EUV mirrors comprise a substrate exhibiting little thermal expansion. A multilayer structure comprising, for example, approximately 20 to approximately 80 double layers comprising silicon (Si) and molybdenum (Mo), or other suitable materials, is applied to the substrate, said layers acting as a dielectric mirror. The European patent document EP 1 829 052 B1 discloses a possible exemplary embodiment of such a reflective multilayer system for the EUV wavelength range.
EUV photolithography masks, or simply EUV masks, additionally have an absorber structure made of absorbing pattern elements on the multilayer structure. In the regions of the EUV mask covered by pattern elements of the absorber structure, incident EUV photons are absorbed or at least not reflected like in the other regions.
EUV masks—or, in general, photomasks—are projection templates, the most important application of which is photolithography for producing semiconductor elements, in particular integrated circuits. Photomasks must be largely error-free, since an error of the mask would reproduce on each wafer during each exposure. Therefore, the highest demands in respect of planar qualities, cleanliness, temperature stability, reflection constancy and freedom of errors are placed on the materials of the optical elements for the EUV range, in particular the photomasks.
In the case of a photomask, it is important that the pattern elements of the absorber structure on the photomask exactly image the structure elements predetermined by the design of the semiconductor element into the photoresist on the wafer. The intended dimension of the structure elements produced in the photoresist by the absorber pattern is referred to as a critical dimension (CD). This variable, or the variation thereof, is an essential characteristic for the quality of a photomask. Freedom of errors for photomasks means that, in this context, the mask upon exposure with the actinic wavelength images an intended dimension within a predetermined error interval onto a wafer, i.e. the CD may only vary within the predetermined error interval. If this condition is satisfied, the photomask has no visible defects or printable defects on a wafer.
Currently, it is not possible to produce substrates and/or multilayer structures for photomasks for the EUV wavelength range which are free from printable defects or errors. The defects considered in this application may have their origin in small local unevenness of the mask substrate (<10 nm deviation from a predetermined thickness), which may propagate through the multilayer structure. Further, local defects within the multilayer structure or particles on the substrate or within the multilayer structure are the cause of impairments of the function of the multilayer structure as a mirror. Below, these defects are referred to as buried defects or defects of the multilayer structure—as is conventional in the art. Currently, there are various concepts for avoiding or at least attenuating the effect of printable defects of EUV masks, which are caused by defects in the multilayer structure.
Thus, after examining the defects of a mask blank, i.e. a substrate with an applied multilayer structure, the pattern elements of the absorber structure can be arranged on the mask blank in such a way that the elements of the absorber pattern substantially cover the printable defects. The article “EUV multilayer defect compensation (MDC) by absorber pattern modification—From theory to wafer validation” by L. Peng, P. Hu, M. Satake, V. Tolani, D. Peng, Y. Li and D. Chen, in “Photomask Technology 2011,” edited by W. Maurer, F. E. Abboud, Proc. of SPIE, Vol. 8166, 81662E-1-81662E-15, describes a simulation tool, with the aid of which the best possible arrangement of an absorber pattern on a defect-afflicted mask blank can be determined very quickly. However, above a certain defect density and depending on the structure of the absorbing pattern elements, this concept quickly reaches its limits.
The obvious procedure for rectifying a buried defect would be to remove the multilayer structure above the defect in a first step, remove the exposed defect in a second step and, thereupon, re-apply the part of the removed multilayer structure in a final step. In practice, this process cannot be carried out on account of the multiplicity of layers in the multilayer structure and the low thickness thereof of approximately 3 nm for the molybdenum (Mo) layers and approximately 4 nm for the silicon (Si) layers and the high demands on the planar properties of the layers or the interfaces thereof.
Instead, U.S. Pat. No. 6,235,434 B1 discloses a method of compensating the amplitude portion of a buried defect by modifying the pattern elements of the absorber structure of an EUV mask in the vicinity of a buried defect. Below, this process is referred to as “compensational repair.” FIG. 1 schematically illustrates the mode of action thereof. A local reduction in the reflectivity which is caused by the locally disturbed surface of a buried defect is compensated by removing parts of the absorber material of adjacent pattern elements of the defect.
The aforementioned patent document describes that it is not the geometric dimension of the buried defect that is intended to be compensated, but its equivalent dimension. The equivalent dimension of a buried defect depends on its spatial orientation in relation to adjacent pattern elements and increases the further the defect is spaced apart from the closest pattern element. Phase defects have a smaller equivalent area than amplitude defects. The position and the equivalent dimension of defect-induced reflection disturbances can be determined by a characterization technique, such as e.g. lithographic printing.
By way of example, the compensational repair is likewise explained in the publication “Compensation for EUV multilayer defects within arbitrary layouts by absorber pattern modification” by L. Pang, C. Clifford, D. Peng, Y. Li, D. Chen, M. Satake, V. Tolani and L. He, in “Extreme Ultraviolet Lithography,” edited by B. M. La Fontaine, P. P. Naulleau, Proc. of SPIE, Vol. 7969, 79691E-1-79691E-14. The modifications of the pattern elements of the absorber structure for compensating local pits or bumps of a mask blank, which are required for defect compensation, are determined with the aid of a simulation tool.
WO 00/34828 describes the repair of amplitude and phase defects of EUV masks on the basis of a change in the pattern elements in the vicinity of the defects.
U.S. Pat. No. 8,739,098 describes a simulation method for determining the dimensions of buried defects of EUV masks and the application of a “compensational repair” for repairing the buried defects.
WO 2016/037851 proposes the subdivision of defects of mask blanks into two classes, wherein the defects of the first class are covered by pattern elements of the absorber structure and the defects of the second class are at least attenuated by the above-described compensational repair.
The article “The door opener for EUV mask repair” by M. Waiblinger, R. Jonckheere, T. Bret, D. van den Heuvel, C. Baur and G. Baralia, in “Photomask and Next Generation Lithography Mask Technology XIX,” edited by K. Kato, Proc. of SPIE, Vol. 84441, 84410F1-84410E-10, 2012, describes the repair of both defects of the absorbing pattern elements and also defects of the multilayer structure of EUV masks, wherein the last-mentioned defects are repaired with the aid of the compensational repair technique.
Further, WO 2011/161243 describes the compensation of defects of EUV masks by producing local changes in the multilayer structure of an EUV mask with the aid of an electron beam.
Moreover, WO 2013/010976 describes the correction of buried defects of EUV masks, wherein the defects are localized by the combined use of an ultraviolet radiation source, a scanning probe microscope and a scanning particle microscope.
Moreover, a further method uses ultrashort laser pulses for local compression of the substrate material of a photomask or mask blank for the purposes of compensating defects of EUV masks. WO 2015/144700 describes the introduction of pixels into a substrate of an EUV mask through the rear side of the substrate, i.e. the side of the mask substrate which lies opposite the multilayer structure.
Finally, in the article “Through-focus EUV multilayer defect repair with nanomachining,” in “Extreme Ultraviolet (EUV) Lithography IV,” edited by P. P. Naulleau, Proc. of SPIE, Vol. 8679, 86791I-1-86791I-4, G. McIntyre, E. Gallagher, T. Robinson, A. C. Smith, M. Lawliss, J. LeClaire, R. Bozak, R. White and M. Archuletta describe that, by way of compensation of the phase disturbances of the multilayer structure induced by local bumps or pits by use of a local removal of part of the defect (in the case of a local bump) or a local deposition of the material on the defect present in the form of a local pit, it is possible to compensate the phase error of these defects.
Defects of the multilayer structure currently represent a main obstacle for the use of photolithography in the EUV wavelength range. Despite the multiplicity of employed methods for correcting defects or attenuating defects, buried defects or defects of the multilayer structure of EUV masks often still cannot be repaired with the required quality.
Moreover, for the purposes of analyzing an EUV mask, it is very important that measurement tools facilitating the reliable determination of the behavior or the operational behavior of an EUV mask are available, without having to carry out time-consuming and expensive exposures of wafers. This point is relevant, in particular, against the background of the further development of EUV masks.
The present invention is therefore based on the problem of specifying methods and apparatuses which improve the examination of EUV masks and thereby also facilitate an improved compensation of defects of EUV masks.
The information provided above is merely to assist the reader in understanding the background of the invention. Some of the information provided in this “Background” section may not be prior art to the invention.